Polymer electrolyte, half cell for electrochemical measurements, and the use thereof

ABSTRACT

A polymer electrolyte for an electrochemical half cell, in particular for a reference half cell, contains a polymer which as a first monomer component contains at least one alkyl methacrylate. The alkyl methacrylate has a substituted alkyl group with from three to seven carbon atoms and at least two substituents. The aforementioned substituents are selected from the group comprising OR 1  and NR 2 R 3 , in which R 1 , R 2  and R 3  are selected from the group comprising hydrogen, methyl, and ethyl, on the condition that the substituted alkyl group contains the substituent OH at most once.

RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. §119 to GermanPatent Application No. 10214035.9 filed in Germany on Mar. 27, 2002, theentire content of which is hereby incorporated by reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The invention relates to a polymer electrolyte, to a half cell,and to uses of the half cell and of the polymer electrolyte.

[0004] 2. Prior Art

[0005] Numerous half cells for electrochemical measurements, such aspotentiometric or amperometric measurements, are known. In particular,such half cells can be embodied as reference electrodes, which are usedin combination with potentiometric or amperometric sensors. In suchreference electrodes, it is imporant that they output as constant areference potential as possible.

[0006] In one type of reference electrodes, there is a fluid referenceelectrolyte, such as an aqueous potassium chloride solution, which canbe brought into contact with a fluid measuring medium via a liquidconnection (also known as a “liquid junction”). To avoid or reduce anunwanted mass transfer between the measuring medium and the referenceelectrolyte, the liquid junction is typically designed as a more or lessporous diaphragm. One disadvantage of such liquid junctions, however, isthat soiling or even plugging up of the pores can occur, which can leadto considerable faulty potentials and possibly interruptions.

[0007] In another type of reference electrodes, instead of thediaphragm, a single opening or a plurality of openings is provided, as aresult of which the aforementioned problem of soiling can be largelyavoided. However, this embodiment means that instead of the otherwiseusual fluid or gel-like reference electrolyte, a non-flowable referenceelectrolyte is used, to prevent the reference electrolyte from flowingout. A polymer electrolyte in the form of a hydrogel, which for instancecontains a saturated aqueous potassium chloride solution and preferablyadditionally suspended potassium chloride is especially well suited forthis purpose.

[0008] In European Patent Disclosure EP 1124132 A1, which is herebyincorporated by reference in its entirety, a reference electrode of thisgeneric type is described which contains a polymer electrolyte thatcontains a polymer on the basis of monomers selected from N-substitutedacrylamides and/or methacrylates. For instance, the methacrylate is amethacrylate with at least two hydroxy groups, and in particular2,3-dihydroxypropyl methacrylate.

SUMMARY

[0009] An object of the invention is to furnish further polymerelectrolytes which are especially well suited for electrochemical halfcells and in particular for reference half cells. Other objects of theinvention are to furnish an improved half cell and to disclose uses ofthe half cell and of the polymer electrolytes.

[0010] The polymer electrolytes of the invention contain a polymer whichas a first monomer component contains at least one alkyl methacrylate.The aforementioned alkyl methacrylate has a substituted alkyl group withfrom three to seven carbon atoms and at least two substituents, and theaforementioned substituents are selected from the group comprising OR₁and NR₂R₃, and R₁, R₂ and R₃ are selected from the group comprisinghydrogen, methyl, and ethyl, on the condition that the substituted alkylgroup contains the substituent OH at most once. By a suitable selectionof the substituents in the aforementioned range, the properties of thepolymer electrolyte, in particular its polarity and hence its resistanceand stability to water or to polar or apolar solvents, can be adaptedaccordingly to the intended area of use. Moreover, the polymerelectrolytes of the invention are distinguished by good resistance andstability to acids.

[0011] The half cell of the invention, which can be used as a componentin potentiometric or amperometric sensors, contains one of the polymerelectrolytes of the invention. The polymer electrolyte of the inventioncan also be used as a solid-phase electrolyte in a battery half cell.

[0012] Exemplary embodiments are directed to the substituted alkyl groupof the alkyl methacrylate intended as the first monomer component.According to exemplary embodiments, the substituted alkyl group is a3-amino-2-hydroxypropyl or a 2-amino-3-hydroxypropyl. Moreover, inaccordance in exemplary embodiments, the substituted alkyl group may bea 3-diethylamino-2-hydroxypropyl, a 3-ethoxy-2-hydroxypropyl, a3-methoxy-2-hydroxypropyl, or a 3- methylamino-2-hydroxypropyl.Furthermore, according to exemplary embodiments, the substituted alkylgroup can be a —CH₂—CH₂OH—CH₂—NR₂R₃ or a —CH₂—CH₂—NR₂R₃—CH₂OH, in whichR₂ and R₃, are as described above. Also, the substituted alkyl group canbe a —CH₂—CH₂OH—CH₂—OR₄ or a —CH₂—CH₂—OR₄—CH₂OH, in which R₄ is a methylor ethyl. However, a mixture of alkyl methacrylates of the type namedabove can also be contemplated as the first monomer component.

[0013] According to exemplary emboidments, the polymer contains, as afurther monomer component, a hydroxyalkyl methacrylate, which ispreferably 2-hydroxyethyl methacrylate and/or 3-hydroxypropylmethacrylate. With this further monomer component, the polarity of thepolymer electrolyte can advantageously be varied, and by a choice of thequantity ratio of the further monomer component to the first monomercomponent, the polarity is adjustable over a wide range.

[0014] Exemplary embodiments can be used in conjunction with a half cellwith a glass housing. Because the polymer contains, as an additionalmonomer component, a silylated alkyl methacrylate, preferably3-(trimethoxysilyl)propyl methacrylate, an adhesion of the polymerelectrolyte to the glass housing is achieved, as a result of which alonger service life of the half cell and in particular better resistanceto pressure and washing out is attained.

[0015] Exemplary embodiments are directed to a polymer electrolyte. Thispolymer electrolyte can additionally contain a concentrated aqueoussolution of a salt or salt mixture, which is indicated for use in polarmeasuring media. The polymer electrolyte can contain a mixture of anorganic solvent and an aqueous solution of a salt and is accordinglypredominantly usable for less-polar measuring media. The organic solventis selected from the group comprising glycerine, ethylene glycol,methanol, ethanol, N-propanol, isopropanol, acetone, and mixturesthereof. It is especially advantageous if the salt is additionally inthe form of a suspension. As a result of the increased salt content, alonger resistance of the half cell to washing out of the salt as aresult of the contact with the measuring medium is achieved. On theother hand, the progressive depletion of the salt caused by the washingout can be readily ascertained visually from the developing washing-outor depletion front, which forms the boundary between a region of thepolymer electrolyte that is clouded by the salt suspension and a regionof the polymer electrolyte that is more clear because the saltsuspension is no longer present there. For instance, the aforementionedsalt is selected from the group comprising potassium chloride, sodiumchloride, lithium chloride, potassium nitrate, potassium perchlorate,sodium formiate, lithium acetate, lithium sulfate, ammonium chloride,methylammonium chloride, dimethylammonium chloride, trimethylammoniumchloride, and mixtures thereof. This salt can, however, also be afurther ionic organic halide, for instance. This salt can also form aredox system.

[0016] The half cell can have an open liquid junction between thepolymer electrolyte and a surrounding medium—as a rule, a measuringmedium or a fluid specimen. This embodiment is possible because thepolymer electrolyte is essentially in solid form and accordingly cannotescape from the open liquid junction. By dispensing with a diaphragm,unwanted interfering potentials in the region of the liquid junction canbe largely avoided.

BRIEF DESCRIPTION OF THE DRAWING

[0017] The sole drawing FIGURE schematically shows a reference electrodefor electrochemical measurements in longitudinal section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The reference electrode shown in the drawing has a tubularhousing 2, made from glass or plastic, whose lower end 4, in the exampleshown, is immersed in a measuring medium 6. The interior of the housing2 is filled with a polymer electrolyte 8, in which a lead-off element 10is immersed. The lead-off element is formed for instance by achlorinated silver wire, which is extended to the outside through anupper cap part 12 of the housing 2. An opening 14 in the vicinity of thelower end 4 serves as liquid junction between the polymer electrolyte 8and the measuring medium 6.

[0019] The polymer electrolyte 8 is advantageously formed in theinterior of the housing 2; first the requisite educts, and in particularthe corresponding monomer components, are introduced into the interior,and after that a polymerization is performed. In this reaction, asolidification takes place, so that the polymer electrolyte 8 formedcannot escape from the opening 14.

[0020] Besides the reference electrode shown in the drawing, otherembodiments are possible. In particular, the reference electrode can becombined with a measuring electrode, for instance a pH electrode, toform a single-rod measurement chain, in a manner known per se. However,still other types of electrochemical half cells can be equipped with thepolymer electrolyte, for instance for amperometric measurements. Thepolymer electrolyte can also be used as a solid-phase electrolyte in abattery half cell.

[0021] Production of Preferred Polymer Electrolytes

[0022] For producing preferred polymer electrolytes, monomer solutionswith a composition shown in Table 1 were mixed with a powder mixtureshown in Table 2 and other additives shown in Table 3 and homogenizedwith cooling at 15 to 20° C. The mixtures thus obtained were introducedinto electrodes with a tubular glass housing. By ensuing heat treatmentof the filled electrodes, polymerization was initiated, which led to theformation of the polymer electrolytes. TABLE 1 Monomer SolutionSubstance Proportion by weight Aminohydroxypropyl methacrylate (AHPMA) 8 to 12% Hydroxypropyl methacrylate (HPMA) 4 to 6% Hydroxyethylmethacrylate (HEMA) 6 to 8% Silylpropyl methacrylate ca. 0.1%N,N,N′,N′-tetramethylethlyenediamine (TEMED) ca. 0.2% Glycerine ca. 30% Powder mixture (according to Table 2) Additives (according to Table 3)Water 15 to 20%

[0023] TABLE 2 Powder Mixture Substance Proportion by weight Potassiumchloride with 2% aerosil 25 to 30% Silica gel H60 3 to 5%

[0024] TABLE 3 Additives Substance Proportion by weightMethylene-bis-acrylamide (MBA) ca. 0.5%  Ammonium persulfate ca. 0.05%2,2′-Azo-bis(2-(2-imidazolin-2-yl)propane) ca. 0.05% dihydrochloride(WAKO 44)

[0025] The first monomer component, designated above asaminohydroxypropyl methacrylate (AHPMA), is a mixture of the two isomers3-amino-2-hydroxypropyl methacrylate and 2-amino-3-hydroxypropylmethacrylate, which can be produced for instance by reacting glycidylmethacrylate (2,3-epoxypropyl methacrylate) with ammonia in an isomerratio of approximately 9:1. WAKO 44 is a radical former that is used asan azo initiator for polymerization reactions. TEMED is a startercompound for the polymerization of acryl and methacryl derivatives.Silica gel and aerosil are used to improve the consistency of thepolymer and to adsorb interfering substances, such as electrode poisons,from the measuring medium.

[0026] The polymer electrolytes described above are distinguished byexcellent stability to acids, water, and both polar and apolar solvents.Accordingly, these polymer electrolytes can be used in the most varioustypes of measuring media.

[0027] Further Exemplary Embodiments

[0028] For particular areas of use, the properties of the polymerelectrolytes can be optimized specifically, as documented by theexemplary embodiments that follow. TABLE 4 Composition of Example 1Proportion by Weight Monomer 3-Amino-2-hydroxypropyl methacrylate 15%3-(Trimethyloxysilyl)propyl  1% methacrylate Cross-linking agent  2%Fillers KCl 25% Silicic acid  5% Solvents Water 22% Glycerine 30%

[0029] TABLE 5 Composition of Example 2 Proportion by Weight Monomer3-Diethylamino-2-hydroxypropyl 20% methacrylate3-(Trimethyloxysilyl)propyl  1% methacrylate Cross-linking agent  5%Fillers KCl 12% Silicic acid  8% Solvents Water 16% Glycerine 38%

[0030] TABLE 6 Composition of Example 3 Proportion by Weight Monomer3-Ethoxy-2-hydroxypropyl 18% methacrylate 3-(Trimethyloxysilyl)propyl 1% methacrylate Cross-linking agent  3% Fillers KCl 20% Silicic acid 5% Solvents Water 20% Glycerine 33%

[0031] TABLE 7 Composition of Example 4 Proportion by Weight Monomer3-Methoxy-2-hydroxypropyl 19% methacrylate 3-(Trimethyloxysilyl)propyl 1% methacrylate Cross-linking agent  3% Fillers KCl 15% Silicic acid 6% Solvents Water 21% Glycerine 35%

[0032] TABLE 8 Composition of Example 5 Proportion by Weight Monomer3-Methylamino-2-hydroxypropyl 14% methacrylate3-(Trimethyloxysilyl)propyl  1% methacrylate Hydroxypropyl methacrylate 4% Cross-linking agent  5% Fillers KCl 20% Silicic acid  5% SolventsWater 15% Glycerine 36%

[0033] All the polymer electrolytes described above here have goodstability to acids; other properties are listed in Table 9. TABLE 9Properties of the Polymer Electrolytes Investigated Example PolarityArea of Use KCl Solution 1 Strong Water; no organic Supersaturatedsolvents 2 Medium- Water; polar Supersaturated strong organic solvents 3Very weak Apolar organic Supersaturated solvents 4 Medium- Polar organicSaturated weak solvents 5 Weak Polar and apolar Supersaturated organicsolvents

[0034] Individually, the following properties were also ascertained. Thepolymer electrolyte of Example 1 is stable to water but not to organicsolvents. The polymer electrolyte of Example 2 is suitable for use withwater and polar solvents. The polymer electrolyte of Example 4, comparedto that of Example 3, has a shorter service life. The polymerelectrolyte of Example 5 is especially well suited, because of itsstability, for use with any organic solvents.

[0035] It will be appreciated by those skilled in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restricted. The scope of the invention isindicated by the appended claims rather than the foregoing descriptionand all changes that come within the meaning and range and equivalencethereof are intended to be embraced thereon.

1. A polymer electrolyte for an electrochemical half cell, comprising: apolymer which as a first monomer component contains at least one alkylmethacrylate, wherein the alkyl methacrylate has a substituted alkylgroup with from three to seven carbon atoms and at least twosubstituents, and the substituents are selected from a group comprisingOR₁ and NR₂R₃, and R₁, R₂ and R₃ are selected from a group comprisinghydrogen, methyl, and ethyl, on a condition that the substituted alkylgroup contains a substituent OH at most once.
 2. The polymer electrolyteof claim 1, wherein the substituted alkyl group is a3-amino-2-hydroxypropyl.
 3. The polymer electrolyte of claim 1, whereinthe substituted alkyl group is a 2-amino-3-hydroxypropyl.
 4. The polymerelectrolyte of claim 1, wherein the substituted alkyl group is a3-diethylamino-2-hydroxypropyl.
 5. The polymer electrolyte of claim 1,wherein the substituted alkyl group is a 3-ethoxy-2-hydroxypropyl. 6.The polymer electrolyte of claim 1, wherein the substituted alkyl groupis a 3-methoxy-2-hydroxypropyl.
 7. The polymer electrolyte of claim 1,wherein the substituted alkyl group is a 3-methylamino-2-hydroxypropyl.8. The polymer electrolyte of claim 1, wherein the substituted alkylgroup is a —CH₂—CH₂OH—CH₂—NR₂R₃ or a —CH₂—CH₂—NR₂R₃—CH₂OH, in which R₂and R₃ are as defined in claim
 1. 9. The polymer electrolyte of claim 1,wherein the substituted alkyl group is a —CH₂—CH₂OH—CH₂—OR₄ or a—CH₂—CH₂—OR₄—CH₂OH, in which R₄ is a methyl or ethyl.
 10. The polymerelectrolyte of claim 1, wherein the polymer contains, as a furthermonomer component, a hydroxyalkyl methacrylate.
 11. The polymerelectrolyte of claim 1, wherein the polymer contains, as an additionalmonomer component, a silylated alkyl methacrylate, preferably3-(trimethoxysilyl)propyl methacrylate.
 12. The polymer electrolyte ofclaim 1, wherein it additionally contains a concentrated aqueoussolution of a salt or salt mixture.
 13. The polymer electrolyte of claim1, wherein it additionally contains a mixture of an organic solvent andan aqueous solution of a salt.
 14. The polymer electrolyte of claim 13,wherein the organic solvent is selected from the group comprisingglycerine, ethylene glycol, methanol, ethanol, N-propanol, isopropanol,acetone, and mixtures thereof.
 15. The polymer electrolyte of claim 12,wherein the salt is additionally in the form of a suspension.
 16. Thepolymer electrolyte of claim 12, wherein the salt is selected from thegroup comprising potassium chloride, sodium chloride, lithium chloride,potassium nitrate, potassium perchlorate, sodium formiate, lithiumacetate, lithium sulfate, ammonium chloride, methylammonium chloride,dimethylammonium chloride, trimethylammonium chloride, and mixturesthereof.
 17. A half cell for electrochemical measurements, comprising: apolymer electrolyte according to claim
 1. 18. The half cell of claim 17,comprising: an open liquid junction between the polymer electrolyte anda surrounding medium.
 19. The use of the half cell of claim 17, as acomponent in potentiometric or amperometric sensors.
 20. The use of thepolymer electrolyte of claim 1, as a solid-phase electrolyte in abattery half cell.
 21. The polymer electrolyte of claim 1, wherein theelectrochemical half cell is a reference half cell.
 22. The polymerelectrolyte of claim 1, wherein the polymer contains, as a furthermonomer component, a 2-hydroxyethyl methacrylate and/or 3-hydroxypropylmethacrylate.